This invention relates to improvements in the production of liquid fertilizers containing nitrogen and sulfur compounds.
The largest segment of nitrogen carriers used in liquid fertilizers are nitrogen solutions. They are used in direct applications to the soil and in the production of other fluid fertilizers. Lists of the many types of nitrogen solutions are given in (USDA Handbook 198: 1965 issue). Nitrogen solutions are designated as Non-Pressure, Low-Pressure and Medium-Pressure solutions.
Ammonium thiosulfate having a content of nutrients expressed, in the conventional manner, as 12-0-0-26S in aqueous solution can be used as a source of nitrogen and sulfur plant nutrients, however, it is expensive for use as a fertilizer. Another possible aqueous fertilizer solution is known commercially as ammonium polysulfide and is made by reacting anhydrous ammonia with elemental sulfur. In aqueous solutions, it can have an analysis as high as 20-0-0-45S. However, it also is an expensive fertilizer and is not compatible with many other nitrogen compounds and solutions or with phosphates (i.e., excessive crystallization and/or solid formation occurs when one attempts to dissolve phosphates or other nitrogen compounds into these solutions). Ammonium polysulfate solutions must also be stored under pressure because they lose ammonia vapors to the atmosphere.
Anhydrous ammonia and aqueous solutions of ammonia are relatively inexpensive and high in nitrogen content (anhydrous ammonia is 82% N). However, these ammonia fertilizers require storage under pressure. When anhydrous ammonia is injected directly into soil from a pressurized container (e.g. at 125 psi or above) it quickly becomes a gas at atmospheric pressure and combines very rapidly with moisture and clay to form ammonium ions, which are stable. However, if aqueous ammonia and anhydrous ammonia are not injected at least about 6" below the soil, or are applied at the soil surface, the loss of nutrient is very rapid, due to volatilization of ammonia into the air.
When nitrogen solutions contain little or no free or uncombined ammonia, they have very low vapor pressure at normal temperatures, and are called non-pressure solutions. Chapter 17 of the "Liquid Fertilizer Manual" of the Natural Fertilizer Solutions Association, Peoria, Ill. lists many of the non-pressure nitrogen solutions. It can be seen from this list that most non-pressure solutions are made from ammonium nitrate or urea or their combinations. Urea-ammonium nitrate formulations are commonly referred to as UAN solutions.
A mixture of ammonium nitrate and urea in the proper proportions gives a non-pressure nitrogen solution, which contains 50 percent or more nitrogen than a saturated solution of either salt alone at the same temperature. The "320" solutions are a mixture of two nitrogen salts which varied between 44-45 percent ammonium nitrate, and 35-36 percent urea, or a similar ratio for the more dilute solutions. The nitrogen content of such a mixture varies from 28 to 32 percent nitrogen with salting out temperatures of 0.degree. to 32.degree. F. They may be used for direct application to soils and crops and in the manufacture of higher nitrogen grades of fluid fertilizer.
One highly useful non-pressurized liquid source of nitrogen is an aqueous solution of urea, ammonia nitrate and minor amounts of ammonia (primarily to adjust pH). The maximum practical nitrogen content in such a solution is about 32% N. One commercial solution is marketed under the trade name UN 32. Such liquid fertilizers can be made directly from carbon dioxide, anhydrous ammonia and nitric acid. Much heat is evolved in this process. More commonly, in a mixing plant such a solution is made by mixing ammonium nitrate (34-0-0) plus urea (46-0-0) plus hot water plus anhydrous ammonia. Another such solution is made by dissolving urea, nitric acid and ammonia. In this process, heat from neutralization of the nitric acid aids in dissolving the urea and evaporates some water, which increases the grade (i.e., makes a more concentrated solution).
It is believed that such solutions have not heretofore been formulated with sulfur compounds to provide a liquid fertilizer having both nitrogen and sulfur as nutrients.
It is frequently desirable to have both nitrogen and sulfur and trace elements, such as zinc, in a liquid fertilizer. It is also desirable, for controlled release of nutrients, to have the nitrogen in a fertilizer present in several forms, e.g., ammonia and urea.
It is also desirable in colder climates to have liquid fertilizers which do not form appreciable crystals (e.g., "salting out") when stored below about 40.degree. F., or in the vicinity of about 32.degree. F. Depending upon the concentration, solutions of ammonium nitrate or urea can have severe crystal formation when stored around about 32.degree. F. For example, an ammonium nitrate solution containing about 20% N can have heavy crystal formation below about 41.degree. F.
Additional data on compositions and salting out of nitrogen solutions can be found in Chapters 17 and 18 of the "Liquid Fertilizer Manual", National Fertilizer Solution Association, Peoria, Ill., said chapters being incorporated herein in reference.